feat: transpile browser bundle

[pi0]

🆒 Your use case
Add a description of your use case, and how this feature would help you.

It is totally fine that we push using modern syntax by default. But mostly for browsers, it might be hard for making utilities that requires users to transpile for browser.

🆕 The solution you'd like

Support target option in siroc.config per-entry. (transpiling with esbuild)

target: {
  main: 'node14',
  browser: 'chrome80'
}

(example above is not esbuild valid targets)

🔍 Alternatives you've considered

• Using babel or a babel based tool like bili
• Forcing users to transpile

ℹ️ Additional info

[danielroe]

nb to self: may be blocked by evanw/esbuild#297

[pi0]

If install size is not much matter, bable env (as an outout plugin) may do the trick for browser 😁